The linear strain of materials, particularly metal, is measured in practice using mechanical or optical extensometers, which measure the elongation of the material between two fixed points on a structure. The measuring points are usually guide marks or dents. These extensometers can also be used to measure the linear strain of non-metal materials, for example, fractures in concrete structures and the like in buildings. Because of their limited accuracy, these extensometers require that the measuring points be located on the material or the structure far from each other. These distances are in the hundreds of millimeters.
Higher accuracy of measurement of the distance between reference points, i.e. the possibility to place the points close to each other, can be achieved by using measuring microscopes. Their principal disadvantages are their rather large dimensions and weight, so in practice they cannot be used to measure the strain of materials of existing metal structures, e.g. bridges, large-diameter pipes and the like, especially if the tested pipes are located in constrained conditions, e.g. in nuclear plants. Another big problem is the reduced accessibility of the tested locations for example in isolated oil pipelines, distribution pipes inside other structures, on bridges and the like. Another disadvantage of measuring microscopes is their proneness to damage and reduction of accuracy.
We also know various types of tension meters, such as mechanical, optical, electrical, acoustic, pneumatic, etc. The disadvantages of mechanical, optical, acoustic and pneumatic tension meters are similar to those of the above-mentioned microscopes. Electrical tension meters eliminate some of the above drawbacks, but their principal disadvantage is that they are only able to work within the range of certain temperature values. In case of higher or lower temperatures it is necessary to specially adapt the electrical tension meters, increasing their acquisition price many times.
Another weakness of tension meters, especially electrical, is their limited lifetime, which is reduced proportionately to the climatic conditions under which the measurements are made.
The existing solutions therefore do not basically allow carrying out field measurements to obtain results which would be comparable to the results of measurements under laboratory conditions.